To measure a molecular weight of a gas, classical methods such as a method in which an intended gas is collected in a container such as a bag and the mass thereof is measured, and a method utilizing a buoyant force of a gas are known. Since a mass is measured mainly by a balance in these methods, a relatively large amount of gas sample is required, and thus these methods require labor and time for the measurement. Furthermore, there is also a problem that it is difficult to handle the case when the composition of an intended gas changes over time.
Besides such classical methods, a mass analyzer configured to ionize molecules and separate the ionized molecules by an electric or magnetic effect is exemplified. However, in this case, a large-sized and expensive apparatus is required. Therefore, it is not practical in many cases to bring a mass analyzer in a place where an intended gas is present and conduct a measurement.
On the other hand, Patent Literature 1 describes an apparatus for measuring a molecular weight of a gas including: a measurement chamber to be filled with a gas to be measured; a vibrator placed in the measurement chamber; an excitation measurement unit that is configured to excite the vibrator and to measure an excitation parameter of the vibrator; a pressure gauge head that is configured to measure a pressure of the gas in which the vibrator is placed; and a temperature gauge head that is configured to measure a temperature of the vibrator, wherein the apparatus includes a calculation unit that is configured to calculate a molecular weight of the gas from the excitation parameter measured in the excitation measurement unit, the pressure measured in the pressure gauge head, and the temperature measured in the temperature gauge head. It is asserted that a molecular weight can be measured by a downsized apparatus on a real-time basis by this molecular weight measurement apparatus.
However, in the case of the molecular weight measurement apparatus of Patent Literature 1, it is necessary to arrange the vibrator with an oscillator so that an alternating-current voltage is applied from a circuit of a driving/measurement unit outside and the frequency thereof automatically becomes identical with a resonance frequency of the vibrator. Furthermore, in the molecular weight measurement apparatus of Patent Literature 1, an excitation parameter measurement apparatus for measuring an applied electric voltage, a flowing electrical current and a frequency, and an amplitude adjusting apparatus for controlling an applied alternating-current voltage are also necessary, and further improvement in downsizing of the apparatus for the simple measurement of a molecular weight is desired.